The subject matter disclosed herein relates generally to temperature estimation in a motor drive and, more specifically, to an improved system for monitoring the temperature of power electronic devices in an integrated motor drive.
As is known to those skilled in the art, motor drives are utilized to control operation of a motor. The motor drive is configured to control the magnitude and frequency of the output voltage provided to the motor to achieve, for example, a desired operating speed or torque. According to one common configuration, a motor drive includes a DC bus having a DC voltage of suitable magnitude from which an AC voltage may be generated and provided to the motor. The DC voltage may be provided as an input to the motor drive or, alternately, the motor drive may include a rectifier section which converts an AC voltage input to the DC voltage present on the DC bus. The motor drive includes power electronic switching devices, such as insulated gate bipolar transistors (IGBTs), thyristors, or silicon controlled rectifiers (SCRs). The power electronic switching device further includes a reverse conduction power electronic device, such as a free-wheeling diode, connected in parallel across the power electronic switching device. The reverse conduction power electronic device is configured to conduct during time intervals in which the power electronic switching device is not conducting. A controller, such as a microprocessor or dedicated motor controller, generates switching signals to selectively turn on or off each switching device to generate a desired DC voltage on the DC bus or a desired motor voltage.
It is also known that each of the power electronic devices has certain inherent power losses, such as conduction losses and switching losses. Thus, as each of the power electronic devices conducts current or as it is turned on and off, power is dissipated as heat within the device. In order to prevent device failure, it is desirable to monitor the junction temperature of the power electronic devices.
Historically, motor drives have been mounted in control cabinets at a location separated from the motor which it is controlling. The motor drives typically utilize power modules which contain the power electronic devices. A power module may include, for example, six IGBTs and their respective free-wheeling diodes (FWDs). The IGBTs and FWDs are enclosed within a plastic housing and terminals are provided to establish an electrical connection between each power electronic device and the DC bus and/or the motor. Also enclosed within each module may be a thermistor to monitor the temperature of module.
However, developments in the power electronic devices used to control the motor have reduced the size of the components. This reduction in size of the power electronic devices along with a desire to reduce the size of the control enclosures have led to placing at least a portion of the motor controller electronics on the motor itself as an integrated motor drive. Specifically, the inverter section, which converts the DC voltage on the DC bus to the AC voltage supplied to the motor, is mounted on the motor. Because the motors are typically located on a machine or within an industrial process line, it is desirable to use an enclosure for the integrated motor drive which has a footprint equal to or less than the area of the surface on the motor to which it is mounted and which has a low profile, and conventional power modules may not fit within the desired enclosure.
As a result, motor controllers have been developed in which individual power electronic devices are mounted within the housing to form an inverter section. The individual power electronic devices may be mounted in a smaller area than traditional power modules. However, by eliminating the traditional power module, the thermistor is no longer present. Providing a separate thermistor within the integrated motor drive has its drawbacks. The thermistor generates an analog signal that is susceptible to interference from modulation of the power electronic devices. Further, the analog signal requires conversion of the signal to a digital signal prior to being input to a controller and isolation of the signal from the controller. Finally, the signal generated is non-linear and requires calibration and compensation within the controller.
Thus, it would be desirable to provide an improved system and method for monitoring the temperature of power electronic devices in an integrated motor drive.